Information recording medium and information recording/reproducing device

ABSTRACT

An information recording medium  101  includes: a transparent substrate  109  having a first surface A and a second surface B opposite to the first surface A; reflectance reduction means  110  provided on the first surface A of the transparent substrate  109;  and an information recording layer  120  provided on the second surface B of the transparent substrate  109.

TECHNICAL FIELD

[0001] The present invention relates to an information recording mediumand an information recording/reproducing apparatus.

BACKGROUND ART

[0002] Examples of conventionally known optical discs to/from whichinformation is recorded/reproduced include CDs, DVDs, etc. Whenrecording/reproducing information to/from a conventional optical disc,an incident angle of light incident on a surface of the optical discdoes not have a great value.

[0003]FIG. 7 illustrates an exemplary configuration including aconventional optical disc 508 and an optical head 513 forrecording/reproducing information to/from the optical disc 508. Theoptical head 513 includes a radiation source 501, a beam splitter 502, acollimating lens 503, an objective lens 506, a hologram 511, and aphotodetector 512.

[0004] When reproducing information recorded on the optical disc 508, alight beam emitted by the radiation source 501 is transmitted throughthe beam splitter 502 and rendered into a parallel beam by thecollimating lens 503. The parallel beam enters the objective lens 506and is converged on an information recording layer 711 of the opticaldisc 508.

[0005] Generally, a semiconductor laser is used as the radiation source501. An emission wavelength of the semiconductor laser is in the rangeof about 780 nm to about 810 nm when the optical disc 508 is a CD and inthe range of about 630 nm to about 670 nm when the optical disc 508 is aDVD.

[0006] The light beam is reflected by the information recording layer711 and transmitted, again, through the objective lens 506 and thecollimating lens 503. Then, the light beam is reflected by the beamsplitter 502 to enter the photodetector 512. In the photodetector 512,an information signal representing information recorded on the opticaldisc 508 and a servo signal for tracking are retrieved from the lightbeam.

[0007]FIG. 8 illustrates a light beam 701 incident on the optical disc508. The light beam 701 includes light 708 vertically incident (i.e., anincident angle is 0°) on a surface of the optical disc 508 and lights707 and 709 incident on the optical disc 508 at an incident angle α. Thelight beam 701 is converged on the information recording layer 711 toform a light spot 712.

[0008] In recent years, for the purpose of recording multimedia dataincluding large-sized dynamic image data or the like, there has been ademand to increase the density of information to be recorded to theoptical disc 508. One method for increasing the density of informationto be recorded to the optical disc 508 is to reduce the size of thelight spot 712 to be formed on the information recording layer 711.

[0009] The diameter φ of the light spot 712 is represented by thefollowing expression (1):

φ=k·λ/(NA)  (1),

[0010] where is λ wavelength of the light beam 701 (i.e., wavelengths oflights 707, 708, and 709), k is a constant, and NA is a numericalaperture of the objective lens 506 (FIG. 7). Constant k is determinedaccording to light distribution at an entrance pupil. Constant k issmall when the light distribution at the entrance pupil is uniform, andconstant k is large when the light distribution at the entrance pupil isnot uniform (e.g., light around the periphery of the entrance pupil isweaker than that in a central portion of the entrance pupil).

[0011] As can be appreciated from expression (1), in order to reduce thesize of the light spot 712, it is necessary to: (1) increase thenumerical aperture NA of the objective lens; (2) uniformly distributelight over the entrance pupil to reduce constant k; or (3) reducewavelength λ of the light beam.

[0012] The wavelength λ of the light beam 701 is determined based on anemission wavelength of the radiation source (e.g., a semiconductorlaser) 501 (shown in FIG. 7). In recent years, a semiconductor laserhaving a short emission wavelength, such as a blue semiconductor laser,is used to reduce the size of a light spot based on the above-mentionedmethod (3). However, in the case of a conventional optical disc, whenthe numerical aperture NA of the objective lens is increased, themaximum possible incident angle of light incident on a surface of theoptical disc becomes large, and thus a reflectance for the light becomeslarge, so that light distribution at the entrance pupil cannot beuniform.

[0013]FIG. 9 shows a relationship between an incident angle of lightincident on the conventional optical disc 508 (FIG. 8) and a reflectancefor the light reflected from a surface of the optical disc. In FIG. 9,dependence of a reflectance on an incident angle is shown with respectto each of S-polarized light (indicated by S in the figure) andP-polarized light (indicated by P in the figure). Note that theP-polarized light refers to polarized light having an electric vectorparallel to a cross section of incidence (i.e., a plane including anormal line of a plane on which light is incident and the incidentlight), and the S-polarized light refers to polarized light having anelectric vector perpendicular to the cross section of incidence.

[0014] From FIG. 9, even when the S- and P-polarized lights have thesame incident angle, respective reflectances for the S- and P-polarizedlights are different from each other. Light incident on the objectivelens 506 (FIG. 7) is generally circularly polarized light, and thus anaverage reflectance for the light beam 701 shown in FIG. 8 from thesurface of the optical disc 508 corresponds to an intermediate value(shown as (S+P)/2 in the figure) between the reflectances for the S- andP-polarized lights. Hereinafter, a reflectance for light refers to theintermediate value ((S+P)/2) between the reflectances for the S- andP-polarized lights unless otherwise indicated.

[0015]FIG. 9 shows that reflectances for the P- and S-polarized lightsare approximately 0% and 20%, respectively, when the incident angle isin the vicinity of 58°. Note that the maximum possible incident angleand the numerical aperture NA of the objective lens 506 are related toeach other such that the maximum possible incident angle is increasedalong with an increase of the numerical aperture NA of the objectivelens 506. The maximum possible incident angle α=58.2° corresponds to thenumerical aperture NA=0.85 of the objective lens 506.

[0016] It is appreciated that in the case of the conventional opticaldisc 508, when the numerical aperture NA of the objective lens 506 (FIG.7) is increased (e.g., when the numerical aperture NA is increased tosuch an extent that the incident angle is greater than 45°), the averagereflectance indicated as (S+P)/2 in the figure is abruptly increased. Alarge reflectance means that light transmitted through the optical disc508 and reaching the light spot 712 is weak. Accordingly, in the case ofthe conventional optical disc 508, when the numerical aperture NA of theobjective lens 506 (FIG. 7) is increased, light around the periphery ofthe entrance pupil (e.g., light having entered the optical disc 508 atthe maximum possible incident angle α and reached the light spot 712)becomes weak. Weakening of light around the periphery of the entrancepupil is equivalent to a relative increase of a value of constant k inthe above expression (1). Accordingly, an effect of reducing the size ofthe light spot 712 cannot be achieved, regardless of the increasednumerical aperture NA.

[0017] As described above, with a conventional optical disc, the lightspot cannot be sufficiently small, and thus it is not possible toincrease a recording density of information.

[0018] The present invention has been made in view of the above problemand an objective thereof is to provide an information recording mediumcapable of increasing the recording density of information. Anotherobjective of the present invention is to provide an informationrecording/reproducing apparatus using the same information recordingmedium.

DISCLOSURE OF THE INVENTION

[0019] An information recording medium of the present inventioncomprises: a transparent substrate having a first surface and a secondsurface opposite to the first surface; reflectance reduction meansprovided on the first surface of the transparent substrate; and aninformation recording layer provided on the second surface of thetransparent substrate, wherein portions of light having entered thefirst surface at an incident angle in the range of 0° to a maximumpossible incident angle, which are transmitted through the transparentsubstrate, are used for recording information to the informationrecording layer or reproducing information recorded on the informationrecording layer, and wherein the reflectance reduction means reduces areflectance for the light which enters the first surface at the maximumpossible incident angle and is reflectedby the first surface, therebyachieving the above objectives.

[0020] The reflectance reduction means may include a transparent thinfilm layer.

[0021] A thickness of the thin film layer may be set so as to minimize asubstantial reflectance for light, which enters the first surface of thetransparent substrate, while being converged, at an incident angle inthe range of 0° to the maximum possible incident angle and is reflectedby the first surface of the transparent substrate.

[0022] The thickness of the thin film layer may be set such that areflectance for light, which enters the first surface of the transparentsubstrate at the maximum possible incident angle and is reflected by thefirst surface of the transparent substrate, is substantially equal tothat for a reflectance for light, which enters the first surface of thetransparent substrate at an incident angle of 0° and is reflected by thefirst surface of the transparent substrate.

[0023] The thickness of the thin film layer may be set so as to minimizethe reflectance for light, which enters the first surface of thetransparent substrate at the maximum possible incident angle and isreflected by the first surface of the transparent substrate.

[0024] The light entering at the maximum possible incident angle mayhave a prescribed wavelength λ and the thickness of the thin film layermay be set so as to substantially satisfy t=λ/(4·n·cosβ), where n is anindex of refraction of the thin film layer and β is an angle satisfyingsinα/sinβ=n when the maximum possible incident angle is α.

[0025] A hardness of the thin film layer may be greater than that of thetransparent substrate.

[0026] The index of refraction of the thin film layer may be smallerthan that of the transparent substrate.

[0027] The maximum possible incident angle may be 50° or more to 72° orless.

[0028] The light entering at the maximum possible incident angle mayhave a prescribed wavelength λ and the reflectance reduction means mayinclude a plurality of structures arranged on the second surface in atwo-dimensional manner, each of the plurality of structures beingsubstantially conical or pyramidal and having a dimension of λ/4 or moreto 2λ or less along the first surface and a height of λ/2 or more to 3λor less from the first surface.

[0029] The plurality of structures may be formed by transcription usinga forming die.

[0030] The plurality of structures may be formed on a film formed of aresin material laminated on the first surface of the transparentsubstrate.

[0031] An information recording/reproducing apparatus of the presentinvention comprises: the above-described information recording medium; aradiation source for emitting light; and light converging means forallowing the light emitted by the radiation source to enter the firstsurface of the information recording medium at an incident angle in therange of 0° to a maximum possible incident angle, thereby achieving theabove objectives.

[0032] The maximum possible incident angle may be 50° or more to 72° orless.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a cross-sectional view of an optical disc 101 (aninformation recording medium) according to Example 1 of the presentinvention.

[0034]FIG. 2 is a diagram showing a relationship between an incidentangle of light incident on the optical disc 101 (FIG. 1) of the presentinvention and a reflectance for the light reflected from the surface Aof a transparent substrate 109.

[0035]FIG. 3 is a diagram for explaining the principle that whenthickness t of a thin film layer 110 is set so as to substantiallysatisfy t=λ/(4·n·cosβ), a reflectance for light, which enters thesurface A of the transparent substrate 109 at the maximum possibleincident angle α and is reflected by the surface A, is minimized.

[0036]FIG. 4A is a cross-sectional view of an optical disk 201 accordingto Example 2 of the present invention.

[0037]FIG. 4B is a perspective view schematically illustrating thesurface structure of the optical disc 101.

[0038]FIG. 5 is a diagram illustrating a configuration of an informationrecording/reproducing apparatus 401 according to Example 3 of thepresent invention.

[0039]FIG. 6 is an enlarged view illustrating a combined objective lens105 and the optical disc 101.

[0040]FIG. 7 is a diagram illustrating an exemplary configurationincluding a conventional optical disc 508 and an optical head 513 forrecording/reproducing information to/from an optical disc 508.

[0041]FIG. 8 is a diagram illustrating a light beam 701 incident on theoptical disc 508.

[0042]FIG. 9 is a diagram showing a relationship between an incidentangle of light incident on the conventional optical disc 508 (FIG. 8) ofthe present invention and a reflectance for the light reflected from asurface of the optical disc.

BEST MODE FOR CARRYING OUT THE INVENTION

[0043] Hereinafter, examples of the present invention will be describedwith reference to the drawings. Herein, identical elements are denotedby identical reference numerals, and repeated description of suchelements may be omitted.

EXAMPLE 1

[0044]FIG. 1 is a cross-sectional view of an optical disc 101 (aninformation recording medium) according to Example 1 of the presentinvention. The optical disc 101 includes: a transparent substrate 109having a surface A (a first surface) and a surface B (a second surface)opposite to the surface A; a thin film layer 110 provided on the surfaceA of the transparent substrate 109; and an information recording layer120 provided on the surface B of the transparent substrate 109. All ofthese elements are held on a base 108.

[0045] The information recording layer 120 may have any configurationcapable of retaining information.

[0046]FIG. 1 shows that light (lights 130, 131, and 132) enters thesurface A of the transparent substrate 109 at an angle in the range of0° to a (the maximum possible incident angle). Portions of the lights130, 131, and 132 transmitted through the transparent substrate 109(corresponding to lights 130 a, 131 a, and 132 a) are converged on theinformation recording layer 120. The transmitted light (the lights 130a, 131 a, and 132 a) is used for recording/reproducing informationto/from the information recording layer 120. Strictly speaking, thelight incident on the optical disc 101 (e.g., the light 130, 131, and132) enters the thin film layer 110 from the surface 125 thereof, andthen the light is transmitted through the thin film layer 110 to enterthe transparent substrate 109 from the boundary (the surface A) betweenthe thin film layer 110 and the transparent substrate 109. However, thethin film layer 110 is extremely thin (e.g., its thickness is smallerthan the wavelength λ of the lights 130, 131, and 132). Accordingly, thesurface 125 of the thin film layer 110 and the surface A of thetransparent substrate 109 are not distinguished from each other and areconsidered as being the same single surface when described inconjunction with incidence and reflection of light unless otherwiseindicated. Therefore, the expression “a reflectance for light incidenton and reflected from the surface A of the transparent substrate 109” isdefined as the ratio of power between reflected light and incident lightwith respect to a single surface.

[0047] When recording information to the information recording layer120, a light beam 133 is modulated according to the information. Thestate of a portion of the information recording layer 120 where a lightspot 134 is formed is changed (e.g., a crystal condition of such aportion is changed) according to the information. As a result, a changein the state of the information recording layer 120 is recorded asinformation to the information recording layer 120.

[0048] When reproducing information recorded on the informationrecording layer 120, the light beam 133 is reflected by the informationrecording layer 120 at a reflectance according to the state of theinformation recording layer 120. An information recording/reproducingapparatus (not shown in FIG. 1, see FIG. 5) detects the intensity of thereflected light to reproduce information.

[0049] The optical disc 101 (an information recording medium) may be areproduction-only recording medium or a recordable and reproduciblerecording medium.

[0050] Note that the maximum possible incident angle α is a constantconforming to a prescribed standard for the optical disc 101. Themaximum possible incident angle α may be, for example, 50° or more to72° or less. The lights 130, 131, and 132 correspond to light emittedfrom the same radiation source and have a prescribed wavelength λdefined by the standard to which the optical disc 101 conforms. Itshould be noted that the term “transparent” described herein refers tothe possession of a property of transmitting light having the prescribedwavelength λ.

[0051] In one embodiment, a polycarbonate resin having a thickness of0.1 mm was used as the transparent substrate 109, and a polycarbonateresin having a thickness of 1.1 mm was used as the base 108. Thetransparent thin layer 110 was formed of a single layer of an acrylicresin material. In FIG. 1, the transparent substrate 109 is provided ononly one side of the base 108. However, the transparent substrate 109may be provided on each side of the base 108. When providing the samestructure at each side of the base 108, the optical disc 101 has asymmetrical structure with respect to the base 108. Consequently, it ispossible to efficiently prevent the optical disc 101 from being tilted.Further, since the information recording layer 120 can be formed on eachside of the base 108, recording capacity can be doubled, therebysatisfying the demand for mass storage.

[0052]FIG. 2 shows a relationship between an incident angle of lightincident on the optical disc 101 (FIG. 1) of the present invention and areflectance for the light reflected from the surface A of thetransparent substrate 109. As can be appreciated by comparing FIGS. 2 to9, the optical disc 101 of the present invention can reduce areflectance for light reflected from the surface A of the transparentsubstrate 109 with respect to an incident angle in the range of 0° to72°. The principle that the reflectance for light reflected from thesurface A of the transparent substrate 109 is reduced holds because thelight reflected by the surface A of the transparent substrate 109 isweakened by interfering with light reflected by the surface of the thinfilm layer 110.

[0053] In FIG. 9, the reflectance for the S-polarized light is increasedalong with an increase of the incident angle, while, in FIG. 2, thereflectance for the S-polarized light is a minimum when the incidentangle has a value indicated by symbol C. The incident angle Ccorresponds to NA=approximately 0.4 to 0.45.

[0054] Similar to the case described above with reference to FIG. 9, anaverage reflectance for the light beam 133 shown in FIG. 1 from thesurface A of the transparent substrate 109 corresponds to anintermediate value (shown as (S+P)/2 in the figure) between reflectancesfor the S- and P-polarized lights. The dependence of the averagereflectance on the incident angle varies according to a thickness of thethin film layer 110 (FIG. 1). The optical disc 101 of the presentinvention reduces the size of the light spot formed on the informationrecording layer 120 (FIG. 1), and therefore a reflectance for light,which enters the surface A of the transparent substrate 109 at a maximumpossible angle α and is reflected by the surface A, is reduced.Accordingly, the optical disc 101 of the present invention is configuredsuch that the thin film layer 110 has a thickness satisfying at leastthe following condition 1.

[0055] Condition 1: the thickness of the thin film layer 110 is set soas to satisfy the condition that provision of the thin film layer 110reduces the reflectance for light which enters the surface A of thetransparent substrate 109 at the maximum possible angle α and isreflected by the surface A. This means that when the maximum possibleincident angle conforming to the standard for the optical disc 101 is,for example, α=58.2° (NA=0.85), a reflectance indicated at point Q₂(FIG. 2) is lower than that indicated at point Q₁ (FIG. 9). When thiscondition is satisfied, provision of the thin film layer 110 intensifieslight around the periphery of the entrance pupil. Accordingly, when thethin film layer 110 is provided, light distribution at the entrancepupil becomes uniform, thereby reducing the diameter of the light spot134 (FIG. 1). Consequently, a recording density of information to berecorded to the optical disc 101 can be increased. Further, informationrecorded on the optical disc 101 at a high recording density can bereproduced.

[0056] In this manner, the thin film layer 110 functions as reductionmeans for reducing the reflectance for light which enters the surface Aof the transparent substrate 109 at the maximum possible angle α and isreflected by the surface A.

[0057] The thickness of the thin film layer 110 is set so as to satisfyany one of the following conditions 2-4, in addition to theabove-mentioned condition 1.

[0058] Condition 2: the thickness of the thin film layer 110 is set soas to satisfy the condition that provision of the thin film layer 110minimizes a substantial reflectance for light which enters the surface Aof the transparent substrate 109 at an incident angle in the range of 0°to the maximum possible incident angle α. Here, the substantialreflectance corresponds to a value obtained by dividing an integralvalue by the range of an incident angle (α-0), where the integral valueis obtained by integrating a reflectance for light, which enters at anincident angle in the range of 0° to the maximum possible incident angleα, over the incident angle. The actual reflectance refers to the ratioof the entire power between the light beam 133 shown in FIG. 1 andportions of the light beam 133, which are reflected by the surface A ofthe transparent substrate 109. Therefore, minimization of thesubstantial reflectance means maximization of power of portions of thelight beam 133 transmitted through the transparent substrate 109. Thisrealizes the optimal efficiency of light entering the surface A of thetransparent substrate 109.

[0059] Condition 3: the thickness of the thin film layer 110 is set soas to satisfy the condition that a reflectance for light entering thesurface A of the transparent substrate 109 at an incident angle of 0°(the light 130 shown in FIG. 1) is substantially equal to that for lightentering the surface A of the transparent substrate 109 at the maximumpossible incident angle α (the light 131 or 132 shown in FIG. 1). Thismeans that when the maximum possible incident angle conforming to thestandard for the optical disc 101 is, for example, α=58.2° (NA=0.85),reflectances indicated at point Q₂ (FIG. 2) and point Q₃ aresubstantially equal to each other. Note that the expression“reflectances are substantially equal to each other” means that adifference between the reflectances is substantially in a normal rangeof design. When this condition is satisfied, the light entering thesurface A of the transparent medium 109 at the maximum possible incidentangle is used as efficiently as that entering the surface A of thetransparent medium 109 at the incident angle of 0°.

[0060] Condition 4: the thickness of the thin film layer 110 is set soas to satisfy the condition that provision of the thin film layer 110minimizes a reflectance for light entering the surface A of thetransparent substrate 109 at the maximum possible incident angle α (thelight 131 or 132 shown in FIG. 1). When this condition is satisfied, thelight entering the surface A of the transparent medium 109 at themaximum possible incident angle is efficiently used.

[0061] The thickness of the thin film layer 110 satisfying theabove-mentioned condition 1 and any one of the conditions 2-4 can bedetermined by calculation or experimentation. For example, theabove-described conditions 1 and 4 are satisfied when thickness t of thethin film layer 110 is set so as to substantially satisfyt=λ/(4·n·cosβ), where n is an index of refraction of the thin film layer110 and β is an angle satisfying sinα/sinβ=n.

[0062]FIG. 3 is a diagram for explaining the principle that whenthickness t of the thin film layer 110 is set so as to substantiallysatisfy t=λ/(4·n·cosβ), the reflectance for light, which enters thesurface A of the transparent substrate 109 at the maximum possibleincident angle α and is reflected by the surface A, is minimized. InFIG. 3, for the purpose of detailedly describing how incident light 132is reflected, the surface 125 of the thin film layer 110 is shown so asto be separate from the surface A of the transparent substrate 109.

[0063] The incident light 132 enters the thin film layer 110 at point Eand is partially reflected by the surface 125 of the thin film layer 110(path D→E→F). Also, the incident light 132 is partially transmittedthrough the thin film layer 110 and the transmitted portions arepartially reflected by the surface A of the transparent substrate 109,transmitted, again, through the thin film layer 110, and exits thesurface 125 (path D→E→G→H→I).

[0064] In order to minimize a reflectance (when the surface 125 of thethin film layer 110 and the surface A of the transparent substrate 109are not distinguished from each other and is considered as being thesame single surface, the reflectance is defined as the ratio of powerbetween light reflected by the same single surface and light enteringthe same single surface), a phase difference corresponding to a ½wavelength is required to be caused between light traveling along thepath D→E→F and light traveling along the path D→E→G→H→I. In this case,the end of a perpendicular line extending from point H to line E-F ispoint H′. Further, the thickness of the thin film layer 110 is t and theindex of refraction thereof is n.

[0065] In order for the phase difference, which corresponds to the ½wavelength, to be caused between the light traveling along the pathD→E→F and the light traveling along the path D→E→G→H→I, the followingexpression (2) is required to be satisfied.

n·2·(distance EG)−(distance H′E)=λ/2  (2)

[0066] From FIG. 3,

[0067] (distance EG)=t/cosβ,

[0068] sinα/sinβ=n, and

[0069] (distance H′E)=2·(distance EG)·sinβ·sinα.

[0070] Accordingly, from expression (2), the following expression (3) issatisfied.

2·n·t·cosβ=λ/2  (3)

[0071] By transforming expression (3), t=λ/(4·n·cosβ) is obtained.

[0072] In the case where condition 4 is satisfied, when an index ofrefraction of the substrate is n₀ and n={square root}n₀ is furthersatisfied, a reflectance for light entering at the maximum possibleangle is substantially zero. Practically, there are few materialssatisfying n={square root}n₀. However, if n<n₀ (i.e., the index ofrefraction of the thin film layer 110 is smaller than that of thetransparent substrate 109) and t=λ/(4·n·cosβ), the reflectance for lightentering at the maximum possible incident angle is always reduced byproviding the thin film layer 110.

[0073] A reduction of a reflectance is preferred not only in that thesize of a light spot can be reduced but also in that stray light causedby reflection can be reduced. A reduction of the stray light caused byreflection is especially preferred when a reproduced signal is weak. Thereduction of a reflectance is also preferred in that light emitted by aradiation source of an information recording/reproducing apparatus(which will be described later with reference to FIG. 5) using theoptical disc 101 can be efficiently used.

[0074] It is preferred that the hardness of the thin film layer 110 isgreater (harder) than that of the transparent substrate 109. In such acase, the thin film layer 110 also functions as a protective coating.

[0075] In order to make the hardness of the thin film layer 110 greaterthan that of the transparent substrate 109, for example, acrylic resin(pencil hardness 2H to 3H) is used for the thin film layer 110 andpolycarbonate resin (pencil hardness B to HB) is used for thetransparent substrate 109. A representative index of refraction ofacrylic resin is 1.48 to 1.50 and a representative index of refractionof polycarbonate resin is 1.55 to 1.58. Accordingly, in such a case, theaforementioned relationship n<n₀ is also satisfied.

[0076] In an objective lens having a high numerical aperture NA, adistance between the lens and an optical disc (working distance WD) isshort. In the case of using such an objective lens, when a surface ofthe optical disc is greatly oscillated or malfunctioning is causedduring retraction of a focus servo, the objective lens may collide withthe optical disc. When the thin film layer 110 made of acrylic resinfunctions as a protective coating, the thin film layer 110 prevents thepolycarbonate resin from being damaged by such collision. The surface ofthe thin film layer 110 made of acrylic resin is smooth andsubstantially frictionless, and thus it is preferred that the thin filmlayer 110 is used for coating. Vapor deposition may be performed on thesurface of the thin film layer 110 for preventing the surface from beingscratched.

[0077] The thin film layer 110 may have a multilayered structure. Whenthe thin film layer 110 has a multilayered structure, the designed widthof the thin film layer 110 is increased for satisfying theabove-described conditions 1-4, and thus a reflectance for theS-polarized light can be substantially zero. However, in view of thecost for producing optical disks it is practically sufficient for thethin film layer 110 to have a single-layer structure.

EXAMPLE 2

[0078] In Example 1, the thin film layer 110 functions as a reflectancereduction means for reducing a reflectance for light which enters thesurface A of the transparent substrate 109 at the maximum possibleincident angle α and is reflected by the surface A, while, in Example 2,another example of the reflectance reduction means is described.

[0079]FIG. 4A is a cross-sectional view of an optical disc 201 (aninformation recording medium) according to Example 2 of the presentinvention. The optical disc 201 includes: a transparent substrate 209having a surface A (a first surface) and a surface B (a second surface)opposite to the surface A; a plurality of structures 203 provided on thesurface A of the transparent substrate 209; and an information recordinglayer 220 provided on the surface B of the transparent substrate 209.All of these elements may be held on a base (not shown).

[0080]FIG. 4A shows that light (lights 130, 131, and 132) enters thesurface A of the transparent substrate 209 at an angle in the range of0° to α (the maximum possible incident angle). Portions of the lights130, 131, and 132 transmitted through the transparent substrate 109(corresponding to lights 130 a, 131 a, and 132 a, respectively) areconverged on the information recording layer 120. The transmitted light(the lights 130 a, 131 a, and 132 a) is used for recording/reproducinginformation to/from the information recording layer 120.

[0081] The optical disc 201 (an information recording medium) may be areproduction-only recording medium or a recordable and reproduciblerecording medium.

[0082] Note that the maximum possible incident angle α is a constantconforming to a prescribed standard for the optical disc 201. Themaximum possible incident angle α may be, for example, 50° or more to72° or less. The lights 130, 131, and 132 correspond to light emittedfrom the same radiation source and have a prescribed wavelength λdefined by the standard to which the optical disc 201 conforms.

[0083] The transparent substrate 209 is formed of glass or a resinmaterial.

[0084] The plurality of structures 203 are arranged on the surface A ofthe transparent substrate 209 in a two-dimensional manner. Eachstructure 203 is substantially conical or pyramidal (e.g.,quadrangular-pyramidal).

[0085] The information recording layer 220 may have any configurationcapable of retaining information.

[0086] Dimension d of a structure 203 along the surface A is preferablyλ/4 or more to 2λ or less, and height h from the surface A is preferablyλ/2 or more to 3λ or less. Here, λ is a wavelength of the light beam 133(i.e., wavelengths of the light 130, 131, and 132). The structure 203 isreferred to as the “subwavelength structure”. The plurality ofstructures 203 function as reduction means for reducing a reflectancefor light which enters the surface A of the transparent substrate 209 atthe maximum possible incident angle α and is reflected by the surface A.

[0087]FIG. 4B is a perspective view schematically illustrating thesurface structure of the optical disc 101.

[0088] Publication 1 shown below describes a reduction of a reflectancedue to the subwavelength structure (Publication 1: Y. Ono, et al.,“Antireflection effect in ultrahigh spatial-frequency holographic reliefgratings”, Appl. Opt. Vol. 26, pp. 1142-1146, 1987). Publication 1describes an example of using an etching method or the like to formstructures arranged on a surface of a glass substrate in aone-dimensional cycle. In the example of Publication 1, light having awavelength of 633 nm is used, and an in-plane cycle of a plane on whichthe structures are arranged is shorter than the wavelength and the depthof the plane is greater than the wavelength. Publication 1 reports acalculation result that the structures realize a reflectance of 0.03 orless at an incident angle in the range of 0° to 60°.

[0089] There is another publication related to the subwavelengthstructure (Publication 2: E. B. Grann, et al., “Optimal design forantireflective tapered two-dimensional subwavelength gratingstructures”, J. Opt. Soc. Am. A Vol. 12, pp. 333-339, 1995), whichdescribes pyramidal subwavelength structures. Such subwavelengthstructures reduce dependence of a reflectance on polarized light. In theexample of Publication 2,light having a wavelength of 4.7 μm is used,and Publication 2 reports a calculation result that a reflectance of0.01 or less (an average reflectance for the P- and S-polarized lights)is obtained at an angle in the range of 0° to 57°. Publications 1 and 2also describe that dependence of such subwavelength structures on awavelength is small.

[0090] Publication 3 (Takahara, Toyota, Okano, Yotsuya, and Kikuta,“Optical characteristics of antireflective structures produced byreactive ion etching”, Proceeding 31a-W-11 of the 47th scientificlecture meeting of the Japanese Society of Applied Physics, 2000)reports that conical subwavelength structures have an effect ofsatisfactorily reducing a reflectance.

[0091] By providing the plurality of structures 203 (subwavelengthstructures) on the surface A of the transparent substrate 209, it ispossible to reduce a reflectance for light reflected by the surface A ofthe transparent substrate 209. Further, since dependence of areflectance on polarized light can be reduced (a difference betweenreflectances for the P- and S-polarized lights can be reduced), lightentering the surface A of the transparent substrate 209 at a large angle(e.g., at the maximum possible incident angle α or an angle in thevicinity thereof) is transmitted through the transparent substrate 209at high transmittance regardless of the polarized direction thereof, andis led to the information recording layer 220. Accordingly, theprovision of the plurality of structures 203 intensifies light aroundthe periphery of the entrance pupil. Consequently, light distribution atthe entrance pupil becomes uniform and the diameter of the light spot134 (FIG. 4A) is reduced. Therefore, a recording density of informationto be recorded to the optical disc 201 can be increased. Further,information recorded on the optical disc 201 at a high recording densitycan be reproduced.

[0092] For example, when forming the transparent substrate 201 (FIG.4A), the plurality of structures 203 may be formed by transcriptionusing a forming die in which a mold of the plurality of structures 203are previously formed by an etching method or the like. According tothis method, the number of steps is not increased as compared to aconventional method for producing optical discs.

[0093] Alternatively, the plurality of structures 203 may be previouslyformed on a film made of a resin material or the like by transcriptionusing a forming die, and the film may be laminated on the transparentsubstrate 209.

[0094] The plurality of structures 203 (subwavelength structures) have aproperty that dependence of a reflectance on a wavelength is small(i.e., an effect of similarly reducing a reflectance for light havingdifferent wavelengths is achieved), and thus the optical disc 201according to Example 2 of the present invention can be preferably usedfor recording/reproducing information thereto/therefrom using lighthaving a plurality of wavelengths.

EXAMPLE 3

[0095] Example 3 is described with reference to an informationrecording/reproducing apparatus including the optical disc 101 (FIG. 1)or optical disc 201 (FIG. 4A) described above in Example 1.

[0096]FIG. 5 illustrates a configuration of an informationrecording/reproducing apparatus 401 according to Example 3 of thepresent invention. The information recording/reproducing apparatus 401includes an optical disc 101 and an optical head 402. The informationrecording/reproducing apparatus 401 records information to the opticaldisc 101 or reproduces information recorded on the optical disc 101. Theoptical disc 101 is the same as that described above in conjunction withExample 1 of the present invention and with reference to FIG. 1.

[0097] The optical head 402 includes a radiation source 104, a beamsplitter 102, a collimating lens 103, a combined objective lens 105, ahologram 111, and a photodetector 112. The combined objective lens 105includes a cemented lens 106 and a front objective lens 107.

[0098] The radiation source 104 is a laser for emitting a violet laserbeam. The central wavelength of the laser beam is 407±20 nm. Thecollimating lens 103 is designed such that defocusing is not caused by awavelength variation within the band.

[0099] In order for the entire optical system including the beamsplitter 102 and the collimating lens 103 to correct chromaticaberration, the collimating lens 103 includes two laminated lenses.

[0100] The photodetector 112 performs a focus servo detection using adetection method referred to as the “spot size detection (SSD)”.According to this method, when a wavelength of a laser beam emitted bythe radiation source 104 varies, a light spot formed in thephotodetector 112 by a ±1-order light beam diffracted by the hologram111 symmetrically moves with respect to a focal point. Accordingly, evenwhen a diffraction angle of the ±1-order light beam at the hologram 111is changed due to a wavelength variation, no errors are caused to adetected servo signal. A method which does not cause an error due to awavelength variation is employed for detecting a tracking error signalby the photodetector 112.

[0101]FIG. 6 is an enlarged view illustrating the combined objectivelens 105 and the optical disc 101.

[0102] The combined objective lens 105 is set so as to have a numericalaperture NA between 0.80 and 0.92. An objective lens having such a highnumerical aperture cannot be configured by a single aspheric lens.Further, in order to correct chromatic aberration, the objective lens isrequired to be configured by using a plurality of lenses made ofvitreous materials having different dispersion characteristics.Therefore, the combined objective lens 105 consists of two groupsincluding three lenses, i.e., the lens 106 including two laminatedlenses and the front objective lens 107. This lens configuration isnecessary for maintaining a high numerical aperture, while correctingchromatic aberration.

[0103] During recording/reproducing of information to/from the opticaldisc 101, the combined objective lens 105 follows the three-dimensionalmovement of the optical disc 101 so as to keep a constant distance(working distance WD) to the optical disc 101. Accordingly, it ispreferred that the objective lens 105 and the optical disc 108 areentirely corrected for chromatic aberration due to color dispersion.Specifically, it is preferred that chromatic aberration of the objectivelens, which is caused due to the lens power and color dispersion,chromatic aberration of the thin film layer 110, which is caused due tocolor dispersion, and chromatic aberration of the transparent substrate109, which is caused due to color dispersion, are entirely cancelled.Note that the information recording/reproducing apparatus 402 accordingto Example 3 of the present invention is set such that WD=0.15 mm.

[0104] Referring again to FIG. 5, the operation of the informationrecording/reproducing apparatus 401 will be described.

[0105] When recording information to the optical disc 101, the radiationsource 104 emits a laser beam modulated according to that information.The light beam is transmitted through the beam splitter 102 and isrendered into parallel light by the collimating lens 103. The parallellight is transmitted through the combined objective lens 105 and entersthe surface A of the transparent substrate 109 of the optical disc 101at an incident angle in the range of 0° to the maximum possible incidentangle α (the maximum possible incident angle α is shown in FIG. 6). Thelight having entered the surface A is transmitted through thetransparent substrate 109 and converged on the information recordinglayer 120.

[0106] When reproducing information recorded on the optical disc 101,the radiation source 104 emits a nonmodified light beam. The light beamemitted by the radiation source 104 is transmitted through the beamsplitter 102 and is rendered into parallel light by the collimating lens103. The collimated light is transmitted through the combined objectivelens 105 and enters the surface A of the transparent substrate 109 ofthe optical disc 101 at an incident angle in the range of 0° to themaximum possible incident angle α (the maximum possible incident angle αis shown in FIG. 6). The light having entered the surface A istransmitted through the transparent substrate 109 and converged on theinformation recording layer 120.

[0107] The light beam is reflected by the information recording layer120 and is transmitted, again, through the transparent substrate 109,the combined objective lens 105, and the collimating lens 103. Thetransmitted light is reflected by the beam splitter 102 and enters thephotodetector 112. In the photodetector 112, an information signalrepresenting information recorded on the optical disc 101 and a servosignal for tracking are retrieved.

[0108] In this manner, the collimating lens 103 and the combinedobjective lens 105 function together as light converging means forallowing the light beam (light) emitted by the radiation source 104 toenter the surface A of the optical disc 101 at an angle in the range of0° to the maximum possible incident angle α. The maximum possibleincident angle α is set so as to be, for example, 50° or more to 72° orless.

[0109] The information recording/reproducing apparatus 401 may beconfigured to perform either recording of information to the opticaldisc 101 or reproducing of information recorded on the optical disc 101or may be configured to perform both. When the informationrecording/reproducing apparatus 401 performs only recording ofinformation to the optical disc 101, the beam splitter 102, the hologram111, and the photodetector 112 can be omitted.

[0110] The information recording/reproducing apparatus 401 according toExample 3 of the present invention uses the optical disc 101 accordingto Example 1 of the present invention, and therefore the combinedobjective lens 105 having a high numerical aperture can efficiently givethe full performance for recording information to the optical disc 101at a high density and/or reproducing information recorded on the opticaldisc 101 at a high density.

[0111] As described above, the optical disc 101 according to Example 1of the present invention can be used for Example 3 of the presentinvention. However, the optical disc 201 according to Example 2 can alsobe used instead of using the optical disc 101.

INDUSTRIAL APPLICABILITY

[0112] As described above, according to the present invention,reflectance reduction means is provided on a first surface of atransparent substrate of an information recording medium. Thereflectance reduction means reduces a reflectance for light which entersthe first surface at the maximum possible incident angle and isreflected by the first surface. This reduces the size of a light spotformed on an information recording layer of the information recordingmedium. As a result, a recording density of information to be recordedon the information recording medium can be increased. Further,information recorded on the information recording medium at a highdensity can be reproduced.

1. An information recording medium comprising: a transparent substratehaving a first surface and a second surface opposite to the firstsurface; reflectance reduction means provided on the first surface ofthe transparent substrate; and an information recording layer providedon the second surface of the transparent substrate, wherein portions oflight having entered the first surface at an incident angle in the rangeof 0° to a maximum possible incident angle, which are transmittedthrough the transparent substrate, are used for recording information tothe information recording layer or reproducing information recorded onthe information recording layer, and wherein the reflectance reductionmeans reduces a reflectance for the light which enters the first surfaceat the maximum possible incident angle and is reflected by the firstsurface.
 2. An information recording medium according to claim 1,wherein the reflectance reduction means includes a transparent thin filmlayer.
 3. An information recording medium according to claim 2, whereina thickness of the thin film layer is set so as to minimize asubstantial reflectance for light, which enters the first surface of thetransparent substrate, while being converged, at an incident angle inthe range of 0° to the maximum possible incident angle and is reflectedby the first surface of the transparent substrate.
 4. An informationrecording medium according to claim 2, wherein the thickness of the thinfilm layer is set such that a reflectance for light, which enters thefirst surface of the transparent substrate at the maximum possibleincident angle and is reflected by the first surface of the transparentsubstrate, is substantially equal to that for a reflectance for light,which enters the first surface of the transparent substrate at anincident angle of 0° and is reflected by the first surface of thetransparent substrate.
 5. An information recording medium according toclaim 2, wherein the thickness of the thin film layer is set so as tominimize the reflectance for light, which enters the first surface ofthe transparent substrate at the maximum possible incident angle and isreflected by the first surface of the transparent substrate.
 6. Aninformation recording medium according to claim 5, wherein the lightentering at the maximum possible incident angle has a prescribedwavelength λ and the thickness of the thin film layer is set so as tosubstantially satisfy t=λ/(4·n·cosβ), where n is an index of refractionof the thin film layer and β is an angle satisfying sinα/sinβ=n when themaximum possible incident angle is α.
 7. An information recording mediumaccording to claim 2, wherein a hardness of the thin film layer isgreater than that of the transparent substrate.
 8. An informationrecording medium according to claim 2, wherein the index of refractionof the thin film layer is smaller than that of the transparentsubstrate.
 9. An information recording medium according to claim 2,wherein the maximum possible incident angle is 50° or more to 72° orless.
 10. (Amended) An information recording medium according to claim1, wherein the light entering at the maximum possible incident angle hasa prescribed wavelength k and the reflectance reduction means includes aplurality of structures arranged on the second surface in atwo-dimensional manner, each of the plurality of structures beingsubstantially conical or pyramidal and having a dimension of λ/4 or moreto 2λ or less along the first surface and a height of λ/2 or more to 3λor less from the first surface.
 11. An information recording mediumaccording to claim 10, wherein the plurality of structures are formed bytranscription using a forming die.
 12. An information recording mediumaccording to claim 10, wherein the plurality of structures are formed ona film formed of a resin material laminated on the first surface of thetransparent substrate.
 13. An information recording/reproducingapparatus comprising: an information recording medium of claim 1; aradiation source for emitting light; and light converging means forallowing the light emitted by the radiation source to enter the firstsurface of the information recording medium at an incident angle in therange of 0° to a maximum possible incident angle.
 14. An informationrecording/reproducing apparatus according to claim 13, wherein themaximum possible incident angle is 50° or more to 72° or less.